Considerations for estimating daily intake values of nonpersistent environmental endocrine disruptors based on urinary biomonitoring data

Human exposure to chemicals may be estimated by back-calculating urinary concentrations resulting from biomonitoring studies if knowledge of the chemical's toxicokinetic properties is available. In this paper, available toxicokinetic data for back-calculating urinary concentrations into daily intake values for bisphenol A (BPA), phthalates, parabens, and triclosan (TCS) are reviewed and knowledge gaps are identified. Human data is evaluated and presented with relevant animal data. Focus is on the recovery of the administered dose, the route of administration, and differences between humans and animals. Two human toxicokinetic studies are currently used to conclude that an oral dose of BPA is recoverable in urine and that no free BPA is present in plasma in spite of several contradicting biominotoring studies. Urinary recovery of an oral dose of phthalates in humans is complicated to assess due to extensive metabolism. In animals using C-14-marked phthalates, near-complete recovery is observed. An oral dose of C-14- marked parabens is also almost completely recovered in animals. In both humans and animals, however, two unspecific metabolites are formed, which complicates the back-calculation of parabens in humans. The recovery of both oral and dermal TCS in humans has been studied, but due to background levels of TCS, the back-calculation is difficult to perform. In conclusion, due to limited data, reasonable estimates of daily intake values based on urinary data are often not possible to obtain. Several knowledge gaps are identified and new studies are suggested. The route of administration used in toxicokinetic studies often does not match realistic scenarios.